Atopic dermatitis is a common chronic inflammatory skin disease. It is a significant medical problem and as much as 15-20% of children are affected by this disease in industrialized countries. Treatment of atopic dermatitis continues to be a challenge. The current model suggests that atopic dermatitis is attributable to a Th2-mediated inflammatory response and T cells, dendritic cells, and mast cells play important roles. This proposal aims at the establishment of the role of a member of the galectin family in atopic dermatitis with the long-term goal of developing a novel therapy for treatment of this disease. Galectin-3 is a member of a family of animal lectins defined by their affinity for 2-galactosides and consensus sequences. It is expressed by a number of cell types, including epithelial cells and various leukocytes. A number of extracellular functions have been demonstrated by using exogenously added galectin-3 and these are associated with its binding to and crosslinking cell surface glycans. However, there is a great deal of evidence that endogenous galectin-3 regulates various cellular functions through intracellular actions. By studying T cells from galectin-3-deficient mice, we have demonstrated that galectin-3 is an inhibitory regulator in Th1 cells and suppresses the T cell receptor (TCR)-mediated Th1 response by promoting TCR downregulation. Galectin-3 is localized intracellularly at the immunological synapse in T cells activated by TCR engagement. We have also demonstrated that galectin-3 suppresses the production of IL-12 by dendritic cells. In addition, we have demonstrated an important role of galectin-3 in mast cells. By yeast two-hybrid screening, we found Alix as a galectin-3-binding partner, which is known to be a component of the endosomal sorting complex required for transport (ESCRT) and the multivesicular body (MVB). We also have other information suggesting that galectin-3 is associated with MVBs and exosomes. In a mouse model of atopic dermatitis, we found that galectin-3 promotes the Th2 response and suppresses the Th1 response and this is in part through the protein's function in T cells and dendritic cells. In this proposal, we plan to test the hypothesis that galectin-3 1) suppresses the TCR-mediated Th1 response by functioning at the IS; 2) is critical for the antigen-presenting function of dendritic cells by suppressing IL-12 production and functions as an exosome-associated protein; 3) promotes allergic skin inflammation through dendritic cells and mast cells (in addition to T cells).